Node editor lagging / unresponsive and imprevisible
Yes!It was my fault from the beginning.
At some point long ago I have changed these settings in Preferences/Input
1.The lag is caused by Drag Threshold and Tweak T. With 3px (minimum allowed) the lag disappears. I supposed &amp; tested that their function is to smooth out drag behavior in mesh editing and slider controls but did not expect to affect the nodes in the editor.
2.The snapping was by Continuos Grab which I use also in 3dMax. And that behavior in node editor is a bug.  


  Drag Threshold
      The number of pixels that a User Interface element has to be moved before it is recognized by Blender.
  Continuous Grab
   Allows moving the mouse outside of the view (for translation, rotation, scale for example).  source: http://wiki.blender.org/index.php/Doc:2.6/Manual/Preferences/Input